The discovery of new therapeutic targets for treating multiple myeloma (MM) and elucidating their underlying molecular mechanisms are necessary for the progress in treatment of this disease. Despite significant recent progress in treating MM, such as the use of proteasome inhibitors, which have a > 70% response rate, the disease inevitably relapses and remains incurable. The oncogene c-Myc is a key driver in MM; it is activated in more than 60% of MM and in nearly 80% of proteasome inhibitor refractory MM. However, drugs that directly inhibit c- Myc do not yet exist, largely because c-Myc is a transcription factor and lacks a defined three- dimensional structure and pockets to which small molecules can bind. In addition to c-Myc, MM cells are strongly dependent on the ubiquitin-proteasome system for survival, as demonstrated by the proteasome inhibitors. In this proposal, we will test the hypothesis that targeting post-translational modifications by the small ubiquitin-like modifier (SUMO) can inhibit c-Myc-dependent pathways and also provide a means for overcoming proteasome inhibitor resistance. The proposed studies are based on our preliminary findings that SUMO modification regulates c-Myc protein levels and activity in MM cells. In addition, our preliminary findings suggest that the mechanisms of bortezomib (proteasome inhibitor) resistance not only could depend on SUMO modification, but also could render proteasome function more dependent on SUMO modification in resistant than in sensitive MM cells. We propose to elucidate the mechanism of how SUMOylation is involved in regulating c-Myc levels and function. In addition, we will elucidate the role of SUMOylation in the poorly understood mechanisms underlying proteasome inhibitor resistance. We will also validate initial in vitro findings in animal models. These studies will be enabled by SUMO E1 inhibitors that we have been developing over the last few years. The proposed studies will likely establish a novel therapeutic target and mechanism for the treatment of MM by offering targeted therapies and by providing therapeutic approaches to overcome resistance to proteasome inhibition. Inhibition of SUMOylation could also address a large population of other cancers that resist conventional therapies by developing drugs that are efficacious against c-Myc-driven cancers.